Lime plaster
A plaster bound with air lime that hardens by carbonation. On older Slovak masonry it is the correct finish because it stays vapour-open, soft and removable.
What is lime plaster and how does it harden?
Lime plaster is a mortar bound with air lime, either hydrated lime or lime putty, with sand as the aggregate and with no cement or only a small proportion of it. It does not harden by hydration the way a cement mortar does. It hardens by carbonation: the lime reacts with carbon dioxide from the air and gradually turns back into calcium carbonate, that is, back into the material it was burnt from.
Everything else about the material follows from that chemistry. Carbonation works from the surface inwards and it is slow, so the plaster gains strength over weeks rather than days and has to go on in several thin coats. The final strength is lower than that of a cement mortar and the modulus of elasticity is markedly lower, which means the material is softer and more willing to move. One point deserves saying immediately, because it is the most common misunderstanding: lime plaster is not an insulant. Its thermal conductivity is roughly 0.7 to 0.9 W/(m·K), an order of magnitude worse than any thermal insulation. It is a finish and a moisture regulator, not a layer that lowers a wall's U-value.
Why does lime plaster belong on older Slovak masonry?
Old walling, whether solid fired brick, stone, unfired earth or mixed masonry, is soft, mobile and almost always damper than a new build. Lime plaster suits it in three respects at once, and the three reinforce each other.
- It is vapour-open, so it lets moisture out of the masonry instead of sealing it in under a hard coating.
- It has a low modulus of elasticity, so it follows movement in the substrate and distributes it into a network of hairline cracks instead of tearing away.
- It is alkaline, which suppresses surface mould growth, although that alkalinity falls over the years.
To those add a property only the next generation will appreciate: lime plaster can be taken off masonry without destroying the bedding mortar and the face of the stone or brick beneath. A cement render in the same situation comes off together with part of the wall. That is why lime is the default in deep renovation of historic fabric, and why heritage authorities ask for it.
Why does a cement render damage an old wall?
Cement mortar has both a high vapour resistance and a high strength, and on a damp wall each of those does its own kind of harm. Moisture rising into the masonry, or entering it from inside, has nowhere to dry to and accumulates behind the render. Where it does manage to evaporate, it deposits salts at the boundary between render and masonry, and the expanding crystals push the render off the wall.
The result is a picture visible on dozens of twentieth-century Slovak houses: a cement plinth peeling away in strips, damp crumbling brick behind it, and mould in the corner of the same wall on the inside. The cement layer still reads as intact from the street at a stage when the masonry behind it is already damaged, which is why the diagnosis usually arrives late. Where a build-up has to dry inwards as well as outwards, the assessment that matters is the one for interstitial condensation, not the appearance of the surface.
| Property | Lime plaster | Cement render |
|---|---|---|
| Hardening mechanism | Carbonation, from the surface inwards, over weeks | Hydration, within days |
| Vapour resistance | Low, the wall can dry through it | High, the wall dries slowly or not at all |
| Strength and stiffness | Low, moves with the substrate | High, tears the substrate instead |
| Behaviour with salts | Salts crystallise in the pores | Salts push the layer off the wall |
| Removal later | Comes off without damaging the masonry | Takes part of the wall face with it |
| Best substrate | Old, damp, soft, uneven masonry | New, dry, flat, strong masonry |
What is a renovation lime plaster and when is it the right answer?
On damp and salt-loaded walls a plain lime plaster is not enough, and renovation lime plasters with a deliberately high porosity are used instead. They do not work by stopping the moisture. The moisture is allowed to evaporate inside the thick porous layer, where the salts are deposited in the pores while the surface stays intact.
It is important to describe this honestly to a client. A renovation plaster extends the life of the surface; it is not a substitute for a missing damp-proof course, and it does not dry a wall that is being continuously fed with water from the ground. The sequence is always the same: find and cut off the source, allow the wall its drying time, and only then apply the finish that lets the residual moisture out.
What does lime plaster cost you on site?
| Site consideration | What to expect | How it is handled |
|---|---|---|
| Application | Several thin coats with waiting time between them | Internal finishing dates move back in the programme |
| Strength gain | Weeks, not days | No heavy fixing or loading of fresh plaster |
| Mechanical damage | Expected in hallways, plinths and internal corners | Local repair, a new coat bonds without a visible joint |
| Frost during curing | Uncured plaster is destroyed by freezing | Work planned into the warm season, no overwintering |
| Substrate preparation | Salts, old cement patches and loose material must go | Survey before pricing, not after starting |
Where does lime plaster not belong?
On a new build in aerated concrete or thermal ceramic blocks the case is different. The substrate is flat, dry and absorbent, the system plasters matched to it are faster and cheaper, and the wall has no historic fabric to protect. Lime is then a deliberate choice made for a vapour-open build-up or for the look of the surface, not the obvious default.
It is also the wrong material where mechanical abuse is certain and repair is unwelcome, and where a surface must be finished and loaded quickly. In both cases the honest answer to a client is that lime buys reversibility and vapour openness, and pays for them with time and with softness. On the right wall that is an excellent trade. On the wrong wall it is an expensive way to get a finish that will be knocked about.
Frequently asked questions
- What is the difference between lime plaster and cement render?
- Lime plaster hardens by carbonation, stays soft and lets vapour through easily. Cement render hardens by hydration, is far stronger and far more vapour-resistant. On a new, dry, flat substrate the strength is an advantage; on old damp masonry the same strength and tightness are what causes the damage.
- Does lime plaster insulate?
- No. Its thermal conductivity is roughly 0.7 to 0.9 W/(m·K), which is an order of magnitude worse than any thermal insulation. Lime plaster is a finish and a moisture regulator, not a layer that lowers the U-value of a wall. Insulating lime plasters exist but they are a compromise on both counts.
- Why does lime plaster take so long to harden?
- It gains strength by reacting with carbon dioxide from the air, and that reaction works from the surface inwards. Strength therefore builds over weeks rather than days, coats have to be thin, and each one needs waiting time before the next. The consequence for a site programme is that internal finishing dates move back.
- Can lime plaster be applied in winter?
- Not safely in Slovak conditions. Plaster that has not yet carbonated is damaged by frost, and because carbonation is slow the vulnerable period lasts far longer than with a cement mortar. Lime plastering is therefore planned into the warm part of the year and an unfinished facade is not left to overwinter.
- Is lime plaster suitable for a new house?
- It can be, but it is rarely the default. On a new wall of thermal blocks the substrate is flat, dry and absorbent, and the system plasters matched to that substrate are cheaper and faster. Lime comes into its own on old masonry, in renovation of historic buildings, and where a vapour-open build-up is being designed deliberately.